8,971 research outputs found
Finally, results from Gravity Probe-B
Nearly fifty years after its inception, the Gravity Probe B satellite mission
delivers the first measurements of how a spinning gyroscope precesses in the
gravitational warping of spacetime.Comment: A Viewpoint article, published in Physics 4, 43 (2011), available at
http://physics.aps.org/articles/v4/43 Submitted to the arXiv by permission of
the American Physical Societ
Post-Newtonian constraints on f(R) cosmologies in metric formalism
We compute the complete post-Newtonian limit of the metric form of f(R)
gravities using a scalar-tensor representation. By comparing the predictions of
these theories with laboratory and solar system experiments, we find a set of
inequalities that any lagrangian f(R) must satisfy. The constraints imposed by
those inequalities allow us to find explicit bounds to the possible nonlinear
terms of the lagrangian. We conclude that the lagrangian f(R) must be almost
linear in R and that corrections that grow at low curvatures are incompatible
with observations. This result shows that modifications of gravity at very low
cosmic densities cannot be responsible for the observed cosmic speed-up.Comment: 10 pages, no figures, revtex
Capture of non-relativistic particles in eccentric orbits by a Kerr black hole
We obtain approximate analytic expressions for the critical value of the
total angular momentum of a non-relativistic test particle moving in the Kerr
geometry, such that it will be captured by the black hole. The expressions
apply to arbitrary orbital inclinations, and are accurate over the entire range
of angular momentum for the Kerr black hole. The expressions can be easily
implemented in N-body simulations of the evolution of star clusters around
massive galactic black holes, where such captures play an important role.Comment: 8 pages, 1 figure, published versio
Constraining Lorentz-violating, Modified Dispersion Relations with Gravitational Waves
Modified gravity theories generically predict a violation of Lorentz
invariance, which may lead to a modified dispersion relation for propagating
modes of gravitational waves. We construct a parametrized dispersion relation
that can reproduce a range of known Lorentz-violating predictions and
investigate their impact on the propagation of gravitational waves. A modified
dispersion relation forces different wavelengths of the gravitational wave
train to travel at slightly different velocities, leading to a modified phase
evolution observed at a gravitational-wave detector. We show how such
corrections map to the waveform observable and to the parametrized
post-Einsteinian framework, proposed to model a range of deviations from
General Relativity. Given a gravitational-wave detection, the lack of evidence
for such corrections could then be used to place a constraint on Lorentz
violation. The constraints we obtain are tightest for dispersion relations that
scale with small power of the graviton's momentum and deteriorate for a steeper
scaling.Comment: 11 pages, 3 figures, 2 tables: title changed slightly, published
versio
Bisimple monogenic orthodox semigroups
We give a complete description of the structure of all bisimple orthodox
semigroups generated by two mutually inverse elements
Post-Newtonian gravitational radiation and equations of motion via direct integration of the relaxed Einstein equations. IV. Radiation reaction for binary systems with spin-spin coupling
Using post-Newtonian equations of motion for fluid bodies that include
radiation-reaction terms at 2.5 and 3.5 post-Newtonian (PN) order O[(v/c)^5]
and O[(v/c)^7] beyond Newtonian order), we derive the equations of motion for
binary systems with spinning bodies, including spin-spin effects. In particular
we determine the effects of radiation-reaction coupled to spin-spin effects on
the two-body equations of motion, and on the evolution of the spins. We find
that radiation damping causes a 3.5PN order, spin-spin induced precession of
the individual spins. This contrasts with the case of spin-orbit coupling,
where there is no effect on the spins at 3.5PN order. Employing the equations
of motion and of spin precession, we verify that the loss of total energy and
total angular momentum induced by spin-spin effects precisely balances the
radiative flux of those quantities calculated by Kidder et al.Comment: 10 pages, coincides with published versio
Post-Newtonian gravitational radiation and equations of motion via direct integration of the relaxed Einstein equations. V. Evidence for the strong equivalence principle to second post-Newtonian order
Using post-Newtonian equations of motion for fluid bodies valid to the second
post-Newtonian order, we derive the equations of motion for binary systems with
finite-sized, non-spinning but arbitrarily shaped bodies. In particular we
study the contributions of the internal structure of the bodies (such as
self-gravity) that would diverge if the size of the bodies were to shrink to
zero. Using a set of virial relations accurate to the first post-Newtonian
order that reflect the stationarity of each body, and redefining the masses to
include 1PN and 2PN self-gravity terms, we demonstrate the complete
cancellation of a class of potentially divergent, structure-dependent terms
that scale as s^{-1} and s^{-5/2}, where s is the characteristic size of the
bodies. This is further evidence of the Strong Equivalence Principle, and
supports the use of post-Newtonian approximations to derive equations of motion
for strong-field bodies such as neutron stars and black holes. This extends
earlier work done by Kopeikin.Comment: 14 pages, submitted to Phys. Rev. D; small changes to coincide with
published versio
The ORNL-SNAP shielding program
The effort in the ORNL-SNAP shielding program is directed toward the development and verification of computer codes using numerical solutions to the transport equation for the design of optimized radiation shields for SNAP power systems. A brief discussion is given for the major areas of the SNAP shielding program, which are cross-section development, transport code development, and integral experiments. Detailed results are presented for the integral experiments utilizing the TSF-SNAP reactor. Calculated results are compared with experiments for neutron and gamma-ray spectra from the bare reactor and as transmitted through slab shields
Propagation Speed of Gravity and the Relativistic Time Delay
We calculate the delay in the propagation of a light signal past a massive
body that moves with speed v, under the assumption that the speed of
propagation of the gravitational interaction c_g differs from that of light.
Using the post-Newtonian approximation, we consider an expansion in powers of
v/c beyond the leading ``Shapiro'' time delay effect, while working to first
order only in Gm/c^2, and show that the altered propagation speed of the
gravitational signal has no effect whatsoever on the time delay to first order
in v/c beyond the leading term, although it will have an effect to second and
higher order. We show that the only other possible effects of an altered speed
c_g at this order arise from a modification of the parametrized post-Newtonian
(PPN) coefficient \alpha_1 of the metric from the value zero predicted by
general relativity. Current solar-system measurements already provide tight
bounds on such a modification. We conclude that recent measurements of the
propagation of radio signals past Jupiter are sensitive to \alpha_1, but are
not directly sensitive to the speed of propagation of gravity.Comment: Revised, 15 pages, 1 figure, accepted for publication in The
Astrophysical Journa
Carter-like constants of the motion in Newtonian gravity and electrodynamics
For a test body orbiting an axisymmetric body in Newtonian gravitational
theory with multipole moments Q_L, (and for a charge in a non-relativistic
orbit about a charge distribution with the same multipole moments) we show that
there exists, in addition to the energy and angular momentum component along
the symmetry axis, a conserved quantity analogous to the Carter constant of
Kerr spacetimes in general relativity, if the odd-L moments vanish, and the
even-L moments satisfy Q_2L = m (Q_2/m)^L. Strangely, this is precisely the
relation among mass moments enforced by the no-hair theorems of rotating black
holes. By contrast, if Newtonian gravity is supplemented by a multipolar
gravitomagnetic field, whose leading term represents frame-dragging (or if the
electrostatic field is supplemented by a multipolar magnetic field), we are
unable to find an analogous Carter-like constant. This further highlights the
very special nature of the Kerr geometry of general relativity.Comment: 4 page
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